Which mode for best interstate MPG??

We just did a drive from Mass. to Florida and back 3,050 miles, averaged 51.3 mpg. We stayed with traffic at 75 - 80 mph.
What I noticed was that when the roads were a coarse pavement the mileage went down. When we drove on smooth roads the mileage went up. In traffic jams the mileage went up, bumper to bumper in electric mode did drive up the mileage. The mileage never went below 47.5 while driving per tank, but it did go back up by the time we filled up.

 

Same here. Sure I did pop on the power mode during the test drive but really since I got the car I just pretty much always leave it in ECO. That way I get the best mileage. Has taught me to be a better, more patient driver as well. Good to know when or if I really need more power I have it; merging with a semi on my but or something. But I really haven't needed to use it. Getting great in town mileage so why change?